How to Fix a Leaky Basement by Adding Under-Slab Drainage


When done correctly, installing under-floor drainage piping in a damp basement can transform the space from an unusable swamp into a dry, comfortable room. The work is difficult and time-consuming, but it can be a panacea when nothing else has worked when done correctly and with the right equipment.

You’ll need to clear the basement first to get to the footings of the walls on all four sides. Anything that could be harmed by dust generated while cutting the concrete floor must be moved or covered.

Find a spot in the cellar where you can dig a pit and surround it with concrete to install a sump pump. Find the farthest, least frequented nook and settle down there. The space under the stairs or in the attic is ideal. A sump pit’s dimensions should be three feet in diameter and the same depth.

You’ll need to see through the slab and chop it out with a demolition hammer to get to the dirt below. At least six inches more space than the final pit size should be allowed for in the saw cut. The concrete must be removed to undercut the slab with a slanted edge. The floor of the pit will be patched using the same concrete that will be used to construct the pit’s walls. Because of the lip created by the undercut, groundwater cannot seep into the new joint and cause the pit walls to collapse. Removing the old concrete and dirt and transporting the surplus materials from the basement to the outdoors need a significant amount of manual labor.

The next step is constructing a plywood cube three feet on a side and about as deep. The pit’s inner walls will take on this shape.

The second stage of this project is just as challenging to complete. The basement floor needs to be sawed out roughly a foot from the base of the walls. There will be unbelievable concrete dust. Close off the stairwell entrance with plastic and tape, crack open the basement windows, and turn on as many exhaust fans as you can to clear the air of dust. Don’t use the kind of fan you’d find in a home. They might easily catch fire from the accumulation of dust on them. Find a local rental shop and pick up a pair of standard exhaust fans. They are cheap for what you get.

Once the saw cuts are complete, you can remove any remaining concrete from the area between the amount and the wall with the demolition hammer. Discarded concrete is taken outside and piled. It’s possible that gravel is all that lies beneath the floor, depending on how the slab was laid. You’ll want to keep this information on hand.

Make sure the trench is at least 12 inches deep. After the holes have been dug, the drainage pipes can be put in. Four-inch perforated black flex pipe, sold in hundred-foot rolls, is the best I’ve found for this purpose. You can also get 90-degree elbows, which are great for navigating tight spaces. A standard hand saw will work just fine for slicing through this pipe. Start with the sump pit and work up the trench, inserting elbows.

You should let the pipe end protrude a few inches through the bottom of the pit you’ve carved out of wood. You’ll have access to the second end of the pipe that leads into the hole once it’s been laid around the basement. If water is pooling in the middle of the cellar and collecting at the walls’ foundations, you may need to extend the drainage system by digging a trench across the room’s center and burying the pipe at its far end in the pit. Wrap the open end with cheesecloth or filter fabric to prevent silt from washing into the tube, as the line will only have one end. The material should be taped down. Covering the short portion of the box encased in the concrete pit wall with felt paper before pouring concrete is another brilliant idea. This will stop wet concrete from leaking into the pipe through any gaps in the line.

Next, using the gravel you dug up, cover the pipes and raise the gravel bed until it’s even with the current floor. Use sufficient two-by-fours to reinforce the pit box against the potential crushing force of the wet concrete. The new concrete should be poured now, and the slab sections should be finished to match the existing floor. As you pour the walls of the pit, prod the concrete to ensure there are no air pockets. Water from your new pipes will likely run into the hole, so the concrete will take several days to cure. Some of the gravel from the pit’s base can be used, but be careful not to bury the plywood too profoundly. Until you’re finished, you can keep the hole as dry as possible by running a hose from the sump pump through a window.

The plywood can be cracked and split for removal by sawing the face with a ground fault circuit (GFI) outlet. Do not give in to the temptation to abandon the removal of the plywood because of how difficult it is. Termites and other pests will soon discover wet wood and feast on it.

Your sump pump should be installed in the pit and piped away from the foundation, preferably to a catch basin or outside. Do not hook up to the public sewage system. Connecting to a public sanitary sewer is prohibited and will ruin a private septic system. Pipes in potentially frosty regions should be buried at least four feet below the surface. Wire mesh pit covers are the final step in this project since they keep out debris and curious tiny hands. A hole only three feet deep can be deadly when young children are around. The wire mesh can be fastened to a floor-mounted wooden frame. You should occasionally run the pump by hand to ensure it is operating well.

Pete, Your Knowledgeable Building Inspector,
Software for Building Inspection and Code Enforcement (BICES)

Pete has been working as a building inspector in the public and private sectors for over 30 years. He has experience in both the office of building design and the field of construction in the Eastern United States, having worked on a wide range of projects from schools to treatment plants, individual residences, and condo projects to major residential landscaping projects. In 2006, he and two other inspectors started Wagsys LLC to provide software for local governments’ building departments, planning boards, and zoning appeals boards.

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